The media learned on October 31 that Eigoro Mogi was going to have surgery to clean out his right elbow some time today (or tomorrow, depending on the source). Mogi started having problems with his elbow in June and later developed right shoulder pain from overcompensation. He decided to have the surgery because the condition was beginning to affect his hitting. He will most likely require a month or two for a full recovery.
[UPDATE 11/3 @ 8:48pm]
The Eagles announced today that Mogi had his right elbow cleaned out at a hospital in Kanagawa on November 2. He will most likely require about four months before he is ready to play again.
Source: Daily Sports 11/3/2017
[UPDATE 1/16/2018 @ 9:55pm]
Mogi swung a bat during practices at Rakuten Eagles Izumi Stadium on January 16. It was the first time he swung a bat since his surgery in November last year. He also started throwing into a net on January 15.
Mogi told reporters that he was at about fifty percent.